Oscillators having an oscillating frequency that can be set over a wide range are normally realized as LC oscillators. In this case, the frequency of the resonant circuit is determined by at least one capacitance and at least one inductance. The oscillating frequency results here from the reciprocal of the product of 2π and the root of the LC product.
In order to adjust the oscillating frequency usually the inductance is constant but the capacitance is made adjustable. Varactor diodes in which the junction capacitance is dependent on an applied DC voltage potential are normally used as adjustable capacitances.
One important area of application for such LC oscillators is mobile radio devices that require radio frequency carrier oscillations in order to perform frequency conversions from radio frequencies to baseband and vice versa.
In order to compensate for losses in the LC resonant circuits, the latter are usually deattenuated by means of a so-called negative impedance. By way of example, cross-coupled MOS field-effect transistors that are fed with a reference current are provided for this purpose. The reference current that feeds an LC oscillator is subject to stringent requirements with regard to its phase noise. With regard to the phase noise, reference-current-fed LC oscillators are very sensitive to the interference and noise inflows in the reference current.
The document Y. Seong-Mo and K. O. Kenneth, Demonstration of a Switched Resonator Concept in Dual-Band Monolithic CMOS LC-Tuned VCO, IEEE 2001 Custom Integrated Circuits Conference, pp. 205–208, IEEE/CiCC 2001, ISBN 0-7803-6591-7, specifies a reference-current-fed LC oscillator as described above.
The described problem area of reducing the phase noise of a reference current source is reduced in the case of an LC oscillator in the document P. Andreani and H. Sjöland, A 2.2 GHz CMOS VCO with Inductive Degeneration Noise Suppression, IEEE 2001 Custom Integrated Circuits Conference, pp. 197–200, IEEE CICC 2001, ISBN 0-7803-6591-7, by virtue of the fact that LC filter structures are provided in series with the current source. However, filters of this type are complicated to realize.